I used to have one of those, and have three Weber Kettles. The only rust you will get is on the grates. The porcelain-enamel coatings on the smoker itself won't rust, unless it is pretty damaged. These things last forever.
Just use a good degreaser to clean the bulk of that crud off. It will not come completely clean, but it will get it clean enough.
As for the grates, Weber Customer Service can get you ANY parts, shipped to your home in a matter of days. They will also keep any Weber products you own on file, so you can easily order parts anytime. I would replace the cooking grate(s). But that's it. That's the part that your food will touch.
Which size WSM is that? I had the 18 inch, which is a lot more expensive than $250. Yours must be the 14-inch.
There is a learning curve to that smoker, but there is also plenty of advice online, including YouTube videos.
This is my collection about ten years ago, with the WSM on the right. That's a rotisserie kit on the red Kettle.
CD